1. Field of the Invention
The present invention relates to a process for electric discharge machining and more particularly, a process for carrying out the electric discharge machining by using a deionized water as working liquid, which contains no rust preventing agent, and an apparatus for carrying out the process.
2. Related Arts
In recent years, a deionized water has been used as working liquid for an electric discharge machining, in lieu of an insulating oil. However, the deionized water has no rust preventing ability for a work to be treated and thus an alkanol amine such as triethanolamine, a fatty amide such as amide oleate or the like has been added to the deionized water to prepare the working liquid (WO 91/04820 and Japanese Patent 5-75524(B)).
A waste liquid containing such a rust preventing agent falls within one of industrial wastes and thus it must entrusted a vendor for its disposal, which increase a working cost of the electric discharge machining and is not preferable from a view point of environmental pollution.
An object of the invention, therefore, is to provide a process for electric discharge machining by using as working liquid a deionized water which contains no rust preventing agent, while preventing generation of rust to a work and an apparatus for carrying out the process.
One of the present inventors has proposed a process and apparatus for improving quality of water by arranging a pair of applying electrodes and a grounding electrode in water to be treated, and applying high-frequency AC voltage to the applying electrodes to decrease oxidation-reduction potential (ORP) of the water and decompose organic and other materials therein (U.S. Pat. No. 5,435,894).
The inventors have applied such a known high-frequency AC applying process to an electric discharge machining to unexpectedly find that generation of rust can be prevented, even if a rust preventing agent shall not be added to a deionized water as working liquid and that although thin rust layer of ferric oxide is formed on surface of a work, but a hard magnetite-like layer is formed under the ferric oxide layer which can be removed by a simple acid dipping operation, whereby the invention has been established.
Therefore, the process for electric discharge machining according to the invention, which comprises a step of arranging a grounding electrode and a pair of applying electrodes in a deionized water, applying high-frequency AC voltage to the applying electrode to decrease oxidation-reduction potential of the deionized water for making it into working liquid, and then carrying out the electric discharge machining operation of a work.
When the process according to the invention is carried out, it is preferable to use the deionized water with ORP of not higher than 200 mV, since the rust preventing effect to the work shall decrease, if ORP of the deionized water is higher.
It is preferable to make the work in a working tank in an electrical grounding state. Although this utilizes a principle of cathodic protection known in the field of DC, but it seems to be effective also in the process for electric discharge machining. The grounding should be made through a resistor of 300-500xcexa9 to prevent transmission of discharge working current to the work.
While, the electric discharge machining apparatus according to the invention, which additionally comprises a high-frequency AC voltage applying device comprising a grounding electrode, a pair of applying electrodes, a DC voltage source, a first and second high-frequency switches connected to the DC voltage source through a variable resistor, respectively, a high-frequency switching commander circuit of a flip-flop circuit connected to the first and second high-frequency switches through a resistor respectively, and a high-frequency oscillation circuit, to apply high-frequency AC voltage to the applying electrodes and decrease oxidation-reduction potential of deionized water as working liquid for a work.
In the electric discharge machining apparatus according to the invention, the grounding electrode and applying electrodes of the high-frequency AC voltage applying device may be arranged in a working tank. Of course, it is possible to prepare the deionized water by treating city water, well water or the like with an ion-exchange resin and transferring the deionized water to the working tank through a storage tank. In such a case, the applying electrodes are arranged in the storage tank and the grounding electrode is arranged in the working tank.
In the high-frequency AC voltage applying device for the electric discharge machining apparatus according to the invention, the voltage of voltage source is 100V, but its effective value is a half or more less and the current value is 500 mA-1 A to ensure operational safety. The frequency of signal given from the high-frequency oscillation circuit to the high-frequency switching commander circuit is in a range of 20-50 KHz. The grounding electrode and each of the applying electrodes were made of steel or stainless-steel plate and titanium plate having adventure finished surface and plated with platinum, respectively.